1. Field of the Invention
This invention relates generally to a closed loop vapor cycle refrigeration system, and more particularly to a refrigeration system wherein the control range of a control parameter is automatically adjusted as the operating conditions change.
2. Description of the Related Art
Refrigeration systems are frequently operated by maintaining a control parameter, such as the superheat, at a set value. However, the ideal or desired superheat setting is different for different fixtures and it also changes (or is dynamic in nature) as the operating conditions or the load on the refrigeration system changes. The design of the fixture (open freezer, closed freezer, upright freezer, etc.) and the load on the refrigeration system determine the desired superheat setting. Furthermore, some refrigeration systems can be operated efficiently at a relatively high superheat setting while others can only be operated efficiently at a relatively low superheat setting.
It is, therefore, desirable to have a refrigeration system in which the set points for the control parameter are automatically changed or adjusted as the operating conditions of the refrigeration system change.
Modern refrigeration systems frequently use electronically controlled expansion valves for controlling flow of the refrigerant to the evaporator coil. Typically, the expansion valve is operated at a single superheat setting and the refrigerant flow rate through the expansion valve is proportional for all values of the superheat. Such proportional control systems are not very responsive to the changes of the operating conditions of the refrigeration system at all operating times, partly because in such systems the percent change in the flow rate at higher superheat values is less than the percent change at lower superheat values. Thus, to effect the same percentage change in the flow of the refrigerant at higher superheat values as at lower superheat values, a greater change in the superheat must occur. Additionally, the prior art refrigeration systems suffer from the problem commonly known as hunting due to the presence of a hysteresis in the expansion valve.
It is, therefore, advantageous to have a refrigeration system which does not exhibit the hunting problem and in which the refrigerant flow rate through the evaporator coil is adjusted so as to cause the refrigeration system to respond more quickly to changes in the operating conditions.
The present invention addresses the above-noted problems and provides a refrigeration system wherein a control region or range for a control parameter is defined as a function of selected parameters of the refrigeration system (selected "system parameters"). The control region is automatically adjusted or changed as the values of the selected system parameters change. The refrigerant flow rate through the evaporator coil is adjusted according to a control response curves defined for the control parameter. An apparatus and method for eliminating the hunting problem associated with the prior art expansion valves and the use of nonlinear flow control response curves for varying the refrigerant flow rate through an expansion valve are disclosed in the parent patent application of this application, Ser. No. 08/001,106, filed Jan. 6, 1993, which application is incorporated herein by reference and is made a part of this application for all purposes.